Sure, but at the run-time cost of both allocating and two passes. I haven't tested this, but there might also be a way to make a closure iterator and wrap it in `toItr(that)`, but then you'd have a function call per summand which would be like 10-15x slower (but one pass & non-allocating, if it even works). (dotCalls for for-loop macros do not currently work yet, either, but I am not sure that is as fundamental a limitation as the other case.)
There may just not be a way to please everyone here. :-(
